Rudolf Diesel himself used vegetable oils for operating the first diesel engines. Like many people after him, he assumed that petroleum would soon run out. The ready availability of petroleum and its fractions in the last century led, in automotive construction, to engines which are optimized with respect to the use of middle distillates of petroleum. In order to achieve a lower level of environmental pollution, considerable improvements were made both in engine technology and in the fuel composition.
Very different considerations, the reduction of the CO.sub.2 emission into the atmosphere and the industrial use of renewable raw materials, once again focused attention on the use of vegetable oils. Their high viscosity makes them unsuitable for use in pure form as fuel in modern automotive diesel engines. The esters of rapeseed oil proved suitable for use in diesel engines. For this purpose, the rapeseed oil is cleaved and the resulting generally unsaturated carboxylic acids are esterified with methanol and the glycerol liberated is isolated. This chemical processing increases the cost of the rapeseed oil methyl ester as diesel fuel to a not inconsiderable extent (DE-A42 09 779, U.S. Pat. No. 5,354,878). Use in winter is limited because the setting point of these esters is between -10 and -20.degree. C.
The use of rapeseed oil methyl esters does not result in a further accumulation of CO.sub.2 in the atmosphere, because this gas was withdrawn from the atmosphere during growth. A great advantage is the biodegradability and the fact that there is virtually no sulfur content. Rapeseed oil methyl ester is therefore suitable as a fuel particularly in forestry and agriculture, in nature conservation and water catchment areas, nature reserves and lakes and rivers. Rape can be cultivated on set-aside areas, reducing the subsidies for agriculture, but the rapeseed oil methyl ester can be used economically as diesel fuel only if the State waives the imposition of a tax to a high degree or entirely. The erection of special plants for the production of rapeseed oil methyl ester will remain limited because the annual production of rape fluctuates greatly. On the one hand, these fluctuations are weather-related or a smaller or greater quantity is cultivated because other crops are preferably produced, depending on the world market price. This irregular production of the vegetable oils occurs more or less with all agriculturally produced oils. Moreover, only the excess quantities not used for food and other industrial purposes are employed for fuels.
It was therefore the object to avoid the above disadvantages of rapeseed oil methyl ester when used as a diesel fuel and to find a way of using vegetable oils in a more suitable form.